Validation of viewstate MAC failed. If this application is hosted by a Web Farm
Two things to consider here.
2. Message Authentication Code (MAC)
1. What is ViewState
When postback happens on webpages, the information of the
webpage will be stored on the ViewState.
After postback, the page controls would get the data from the ViewState.
2. What is Message Authentication Code (MAC)
When doing the page redirection or postback, there is a possibility that
the data in the ViewState can be tampered.
Can be called as HACKED ...!!!!!! ...
In oreder to protect this tampering, the Viewstate data
is protected by Message Authentication Code (MAC).
This Error mainly occurs When the MAC validation for the Viewstate fails.
When there is not provided any machine key on the web.config of each IIS application then
Asp.Net automatically generate cryptographic key for each application and
stores the key in the HKCU registry.
When mismatch happens between the applications on the auto generated key, this error happens.
On the web.config of your asp.net application, add the following code for machine key.
For Creating the machine Key, Please follow link. How To Generate Machine Key On IIS
For Video of Generating Machine Key : Youtube - How To Generate Machine Key On IIS
This would definitely resolve the issue.